1. Field of the Invention
The present invention relates to a method and apparatus for estimating the running state of a vehicle by estimating the condition of a road surface or the running state of each tire while running, an apparatus for controlling a vehicle based on the estimated running state of a vehicle, and a tire wheel comprising the above vehicle running state estimation apparatus and a power generating unit for activating this apparatus.
2. Description of the Prior Art
In recent years, it has been desired that the relationship between each tire and the surface of a road which is the most important factor for the safe running of a vehicle, specifically, the ground contact state of the tire typified by a friction coefficient between the tire and the surface of a road (road surface friction coefficient) or the condition of a road surface, or the running state of the tire such as the distortion and air pressure of the tire should be estimated with high accuracy and fedback to vehicle control. That is, if the above ground contact state and running state of the tire can be estimated in advance, before the operation of avoiding a risk such as braking or steering is taken, high-level control of an ABS brake will be made possible and further improvement of safety will be expected. The driver can carry out deceleration operation earlier if he is informed of the risk of the condition of a road surface while running, whereby a reduction in the number of accidents can be expected.
To estimate a road surface friction coefficient, there are proposed a method of estimating a road surface friction coefficient making use of the fact that the uniformity level of each tire which is a physical quantity indicative of a change in the revolution speed of each wheel is changed by the size of a road surface friction coefficient (Japanese Laid-open Patent Application No. 2000-55790) and a method of estimating a road surface friction coefficient making use of the fact that the horizontal-direction vibration of each tire having a toe angle is detected by attaching an accelerometer to a lower arm for connecting the front wheels and the vehicle body and this vibration level is changed by a road surface friction coefficient (Japanese Laid-open Patent Application No. 6-258196).
However, in the above method of estimating a road surface friction coefficient from the uniformity level of the tire, the uniformity is deteriorated by the formation of a flat spot in the tire and in the course of recovery from this, accurate estimation is difficult.
Meanwhile, in the above method of estimating a road surface friction coefficient from the horizontal-direction vibration of the front wheels having a toe angle, the measurement accuracy is low when the slip angle of the tire is taken completely null or large.
There is also proposed a method of estimating a road surface friction coefficient from transmission characteristics between acceleration below a spring which is acceleration in the vertical direction of each wheel and acceleration above the spring which is acceleration in the vertical direction of the vehicle body (Japanese Laid-open Patent Application No. 11-94661). This method has such an advantage that the road surface friction coefficient on a straight road for which almost no steering action is carried out can be estimated because steering force is not used for the estimation of a road surface friction coefficient. However, as the road surface friction coefficient is estimated from vibration transmission characteristics between two points through a suspension unit having large buffer characteristics such as a spring or damper, the road surface friction coefficient is readily affected by the uneven surface of the road. For instance, as vibration under a spring is large on a rough road such as a road covered with snow, the difference in vibration level between vibration above the spring absorbed by a suspension and vibration below the spring becomes large, thereby making it impossible to estimate a road surface friction coefficient accurately.
Meanwhile, the internal pressure of the tire is also an important factor for the running condition of the tire. Stated more specifically, the ground contact state of the tire and the running state of the tire are accurately estimated from the distortion state or vibration level of the tire while rolling and grip performance is improved or riding comfort is improved by increasing the ground contact area or rigidity of the tire is reduced to reduce the internal pressure of the tire when the grip performance of the tire is reduced on a wet road or road covered with iced snow or when the vehicle runs on a rough road. Conversely when the vehicle runs at a high speed or a hydroplaning phenomenon occurs, the running fuel cost must be improved or the recovery of steerability must be promoted by increasing the internal pressure of the tire.
However, since a sensor, which is ground contact state detection means for measuring the distortion state or vibration level of the tire while rolling, requires a electric power source, the power must be supplied to the above sensor. Further, when an apparatus for estimating or controlling the condition of a road surface or the running state of the tire based on the output of the above ground contact state detection means and a radio unit for transmitting an output signal from road surface condition estimation means or the like to the vehicle body are mounted to the tire, a electric power supply to the above apparatus and radio unit is necessary.
For the power supply to the tire as a rotor, electromotive force is transferred through a slip ring or generated by electromagnetic induction making use of relative movement between vehicle body and the tire may be used. However, the structure of the vehicle body must be changed for means of supplying power to these, thus boosting costs.
Although it can be said that it is the most realistic method to load batteries which are to be exchanged, there remain such problems as the troublesome exchange and service life of the batteries.
The development of a system which estimates the running state of a vehicle such as the condition of a road surface or the running state of each tire accurately, supplies information on the running state of the vehicle to the vehicle and the driver and controls the characteristics of the tire using the above information to provide a more safe or more comfortable running state has been desired.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a method and apparatus for estimating the running state of a vehicle such as the condition of a road surface or the running state of each tire while running accurately, a vehicle control apparatus for improving the safety of a vehicle by feedback controlling the running state of the vehicle based on the estimated condition of a road surface or the estimated running state of each tire, and a tire wheel comprising the above vehicle running state estimation apparatus and a power generating unit for activating the apparatus.